The use of electrical measurements for gathering of downhole information, such as logging while drilling (“LWD”), measurement while drilling (“MWD”), and wireline logging system, is well known in the oil industry. Such technology has been utilized to obtain a great quantity of geologic information regarding conditions and parameters encountered downhole. It is important to determine geologic information with a high degree of accuracy for drilling efficiency. For example, as known in the prior art, the formation containing hydrocarbon (such as crude oil or gas) usually has higher resistivity than the formation containing water. It is preferable to keep the borehole in the pay zone (the formation with hydrocarbons) as much as possible so as to maximize the recovery.
Geologic information typically includes formation resistivity (or conductivity; the terms “resistivity” and “conductivity”, though reciprocal, are often used interchangeably in the art), dielectric constant, data relating to the configuration of the borehole, etc. Borehole images could help geologists and geophysicists define the structural position of reservoirs and characterize features, such as fractures and folds. However, the nonconductive environment prohibits the use of conventional micro-resistivity imaging devices. In such circumstance, either oil-based mud resistivity imaging devices have to be used or the mud must be changed at great expense and inconvenience to the operator. Therefore, the use of nonconductive (e.g. oil-based and synthetic) mud in drilling process, which can be utilized to reduce drilling risks and improve drilling efficiency, has become more and more popular nowadays.
FIGS. 1A and 1B show a side view and a front view of an oil-based mud resistivity imaging device making four-terminal resistivity measurements as known in prior art. The sensor pad 100 can be deployed against the borehole wall 120 for measuring the resistivity of formation 102 near the borehole. The sensor pad 100 includes two current electrodes 104 and 106 and several voltage electrodes 108 and 110. A standoff 112 would possibly be situated between the formation 102 and the sensor pad 100. The standoff 112 can be filled with nonconductive fluid, such as an oil-based mud or mix of it and other materials form the borehole, present in the borehole whiling drilling.
In operation, the current electrodes 104 and 106 are used to conduct electric current (I) through the formation 102. The pair of voltage electrodes 108 and 110 is used to measure the voltage difference (dV) between them. According to the Ohm's Law, the resistivity of the small interval between the pair of voltage electrodes 108 and 110 of the formation 102 can be computed as follows,
                    Rt        =                  k          ⁢                      dV            I                                              (        1        )            
where k is a geometrical factor.
However, although the oil-based mud resistivity imaging devices have been used commercially, the imaging quality still strongly depends on the borehole environment (e.g. mud film or mud cake thickness, rugosity of the borehole wall etc.). The rugosity of the borehole wall would cause tilt of the sensor pad. As a result, the measurement of the voltage difference between the pair of voltage electrodes would be affected.
Furthermore, the capacitive coupling between the current and voltage electrodes and the formation would be significant when the oil-based mud resistivity imaging device is operated at relatively low frequency (e.g., a few kHz). When the oil-based mud resistivity imaging device is excited at low frequency, most of voltage drops would occur in the mud film, instead of the formation, due to limited current being able to penetrate the very resistive oil film to reach the formation.
Also, in the case where the formation water is relatively fresh or variable, the resistivity-based methods at a relatively low frequency are difficult because of the small and uncertain contrast between hydrocarbons and water. Should being under this situation, dielectric properties can provide an alternative means of evaluating water distribution since the dielectric constant of water differs nearly an order of magnitude from the dielectric constants of other formation constituents. In addition, dielectric permittivity is also of interest in evaluating zones where the water salinity is unknown, as might be the case in secondary recovery projects where water injection has altered the formation water.
As described above, a need exists for an improved apparatus and method for an improved excitation for oil-based mud resistivity imaging.
A further need exists for an improved apparatus and method for minimizing the tilt effect and enhancing transmission and reception.
A further need exists for an improved apparatus and method for making both formation resistivity and dielectric permittivity measurements.
The present embodiments of the apparatus and the method meet these needs, and improve on the technology.